In 1976, professor Kaminsky of Germany reported olefin olefin polymerization using a methylaluminoxane (MAO) compound obtained through partial hydrolysis of trimethyl aluminum as a co-catalyst and a zirconocene dichloride compound as a catalyst (A. Anderson, J. G. Corde, J. Herwig, W. Kaminsky, A. Merck, R. Mottweiler, J. Pein, H. Sinn, and H. J. Vollmer, Angew. Chem. Int. Ed. Engl., 15, 630, 1976).
Later, Exxon filed a patent regarding olefin polymerization using a metallocene compound having a variety of substituents in a cyclopentadienyl ligand (U.S. Pat. No. 5,324,800).
Such a metallocene catalyst has a homogeneous active site, thus advantageously causing a narrow molecular weight distribution of polymers, securing easy copolymerization and homogeneous distribution of second monomers, and enabling control of the steric conformation of polymers during propylene polymerization. In particular, only isotactic polypropylene can be prepared by using conventional Ziegler-Natta catalysts, while various polypropylenes such as isotactic, syndiotactic, atactic and hemiisotactic polypropylenes can be stereoregularly prepared by using metallocene catalysts. For example, syndiotactic polypropylene synthesized using metallocene has a low crystallinity, proper rigidity and hardness, excellent transparency and high impact resistance. That is, metallocene catalysts are actively researched, since they have advantages of enabling steric control in the process of preparing polyolefins and facilitating control of physical properties of polymers.
However, olefin polymerization using such a homogeneous catalyst has disadvantages of difficulty in maintaining the form of polymers in the case of a gas phase process or a slurry phase process and of the necessity of excess MAO in order to maximize the activity of metallocene catalysts. In order to solve these disadvantages, metallocene catalysts should be used in a state where they are supported on a proper carrier. Such a supported metallocene catalyst can advantageously control the form of prepared polymers, control the molecular weight distribution depending on the intended use, improve the apparent density of obtained polymers and reduce a fouling phenomenon in a reactor.
Known general methods for preparing a supported metallocene catalyst are a method for preparing a supported metallocene catalyst by physically and chemically bonding a metallocene compound to a carrier and coming the bonded compound into contact with aluminoxane; a method for preparing a supported metallocene catalyst by supporting aluminoxane on a carrier, followed by reaction with a metallocene compound; and a method for preparing a supported metallocene catalyst by bringing a metallocene compound into contact with aluminoxane and then supporting the resulting compound on a carrier. Such a supported catalyst should maintain a single active site catalyst structure even after the supporting process so that it can exhibit high activity and copolymerization efficiency, comparable to a homogeneous catalyst, and it should not be separated from the carrier in the polymerization process in order to prevent fouling in the reactor. Further, the particle size, size distribution and apparent density of polymers depend on particle shape and mechanical properties of supported catalyst.
Korean Patent No. 0404780 discloses a metallocene compound having a silacycloalkyl substitutent and a supported catalyst using the same. In accordance with this patent, the reactor fouling phenomenon may occur, since the catalyst is separated from the carrier, when used in a gas phase process or a slurry phase process.
Meanwhile, Japanese Patent No. 6-56928 discloses a method for preparing a supported metallocene catalyst by supporting ligands on the surface of a carrier through chemical bonding, and bonding the ligands to a metal. This method has disadvantages of complicated catalyst preparation process and the difficulty in supporting a great amount of catalyst on a carrier.
Of the aforementioned various methods for preparing supported catalysts, the method comprising supporting aluminoxane, followed by reaction thereof with a metallocene compound, is the oldest method for preparing a heterogeneous single active site catalyst. For example, silica reacts with an aluminoxane solution, the reaction solution is filtered, and the filtrate reacts with zirconocene dissolved in toluene or an aliphatic hydrocarbon solvent to prepare a supported catalyst, which can be immediately used for ethylene polymerization or copolymerization in a gas phase process or a slurry phase process, without any further treatment. This supporting method enables preparation of a single-phase catalyst which exhibits relatively high activity and eliminates the necessity of using additional aluminoxane for a polymerization reactor, since the co-catalyst is physically or chemically bonded to the surface of carrier and the catalyst is ionically bonded to the co-catalyst, similar to a homogeneous system catalyst, thus readily applying to a conventional slurry or gas phase process. However, this method is disadvantageous in that the separation of catalyst cannot be completely prevented and, in some cases, fouling may thus occur, and aluminoxane which can be bonded to silica is limited, thus inevitably involving a limitation of a metallocene compound which can be bonded.
WO 2002/040549 discloses an olefin polymerization catalyst comprising a metallocene compound, a supported activator such as MAO, an ionization activator such as dimethylanilinium tetra(pentafluorophenyl)borate and triphenylcarbeniumtetra(pentafluorophenyl)borate and a carrier. Japanese Patent Publication No. 2008-121027 discloses a catalyst for preparing an olefin polymer, comprising a carrier such as silica, methylaluminoxane, a transition metal compound such as bis(indenyl)zirconium dichloride and [PhNMe2H] [B(C6F5)4]. U.S. Patent Publication Application No. 2006/0116490 discloses a metallocene catalyst for olefin polymerization comprising a combination of a carrier and an ionic compound such as aluminoxane and tetrakis (pentafluorophenyl) borate, as a co-catalyst, and a metallocene compound. Akihiro Yano disclosed an ethylene polymerization catalyst comprising dimethylanilinium tetrakis(pentafluorophenyl)borate (Me2PhNH.B(C6F5)4)/triisobutylaluminum (i-Bu3Al) co-catalyst and a metallocene compound (Journal of Molecular Catalysis A: Chemical 156—2000.133-141). However, these catalysts have a disadvantage of low activity.